1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which acquire plural images corresponding to mutually different parameters such as TRs and/or TEs with regard to a same object.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on NMR signals generated due to the excitation.
In magnetic resonance imaging, various species of parameter image such as a longitudinal relaxation (T1) weighted image (T1WI), a transverse relaxation (T2) weighted image (T2WI), a proton density weighted image (PDWI), a FLAIR (fluid attenuated IR) image, a diffusion weighted image (DWI) or a perfusion weighted image (PWI) of blood flow in a capillary vessel is imaged by changing a parameter serving as an imaging condition such as a repetition time (TR), an echo time (TE), an inversion time (TI) in case of a scan under an inversion recovery (IR) method, a b-factor indicating an intensity of a MPG (motion probing gradient) pulse applied in a diffusion weighted imaging (DWI), an application and an intensity of a pre-pulse to control a contrast.
FIG. 1 is a chart showing an example of pulse sequence for acquiring a DWI and a non-DWI of a same object in the conventional MRI apparatus.
In each of (A) and (B) of FIG. 1, ECHO, Gr and Ge denote echo data (magnetic resonance signals) to be acquired, gradient magnetic fields for RO (readout) and gradient magnetic fields for PE (phase encode).
As shown in FIG. 1, respective scans are performed according to two different sequences (A) and (B) in case of acquiring a DWI and non-DWI from a same object.
That is, in case of acquiring a DWI as shown in FIG. 1 (A), echo data is acquired according to an EPI (echo planar imaging) sequence with application of a MPG pulse by setting a b-factor to bn which is non-zero, for example. More specifically, Nr echo signals are respectively acquired with a sampling pitch Δt, and an echo signal train consisting of the Nr echo signals is acquired at an echo train spacing (ETS) Ne/m times per single shot. Then, Ne echo signal trains are acquired by m shots of data acquisition.
In contrast, in case of acquiring a non-DWI as shown in FIG. 1 (B), echo data is acquired by an EPI sequence without application of a MPG pulse, i.e., with setting a b-factor to b0 which can be regarded as zero, as with DWI acquisition.
FIG. 2 is a diagram showing a method for arranging the echo data, acquired by the pulse sequence shown in FIG. 1, in k-space (Fourier space).
In each of (A) and (B) of FIG. 2, the abscissa axis denotes the readout direction Kr in k-space and the ordinate axis denotes phase encode direction Ke in k-space. As shown in FIG. 2, echo data (bn data) acquired by the DWI sequence with b=bn and echo data (b0 data) acquired by the non-DWI sequence with b=b0 are arranged in individual k-spaces respectively. In the case where the number of shots m=3 for example, Ne/3 echo signal trains are acquired three times, and the number of data in a phase encode direction becomes Ne. Since a single echo signal train consists of Nr echo signals, the number of data in a readout direction becomes Nr.
In such magnetic resonance imaging, techniques to shorten an imaging time include the echo train imaging that acquires k-space data separately. The echo train imaging acquires multiple echo signals corresponding to mutually different TEs with changing a phase encode after a single excitation by a FSE (fast spin echo) sequence or an EPI sequence, and arranges the echo signals corresponding to the mutually different TEs on corresponding frequencies in k-space to acquire an image.
Further, techniques to improve a time resolution include a technique called keyhole. The keyhole is frequently used in a dynamic imaging using contrast medium (see, for example, “Glossary of Clinical Magnetic Resonance Imaging”, page 107 and page 333, Chief Editor: Kazuhiro Tsuchiya, Editor: Kazuyuki Ohgi, Medical View Co., Ltd). Specifically, while data is acquired over entire frequency band before injection of contrast medium, data in only a low frequency region is acquired after the injection of the contrast medium and the data acquired before the injection of the contrast medium is used for data in a high frequency region or the time resolution in the high frequency region is reduced. Consequently, the time resolution can be improved without reducing the spatial resolution or a width of FOV (field of view).
However, in a MRI apparatus, various images of a same object are imaged such as a T1WI, a T2WI, a PDWI, a FLAIR image, a DWI and a PWI with mutually different parameters including a TR and a TE. Since it is necessary that the number of multiple images equivalent to the number of different parameters are separately acquired, there is a problem that an imaging time is long in a MRI apparatus compared to an image diagnostic apparatus such as an X-ray CT (computed tomography) apparatus.